An installation for dispensing liquid such as fuel for motor vehicles generally comprises means for dispensing said liquid essentially comprising volumeters fitted with pumps adapted to cause the fuel to flow with a liquid flowrate Q.sub.L between a storage tank and the fuel tank of the vehicles. The volumeters also include a liquid measuring device connected to a pulse generator enabling a computer to establish the volume and the price of the fuel delivered, which are shown in the clear on a display with which the volumeters are equipped.
If the hydrocarbon vapor emitted is to be recovered, said installation includes recovery means adapted to cause said vapor to circulate with a vapor flowrate Q.sub.V along a pipe between the vehicle fuel tank and a recovery tank, for example the storage tank, the vapor flowrate Q.sub.V being controlled by a parameter G characteristic of said recovery means so as to maintain between the vapor flowrate Q.sub.V and the liquid flowrate Q.sub.L a relation of proportionality Q.sub.V =kQ.sub.L with k equal to or close to 1.
Said recovery means usually comprise a pump aspirating the vapor from the fuel tank in order to return it to the hydrocarbon storage tank. The characteristic parameter G is the rotation speed w of said pump which is controlled by the pulse generator of the dispensing means.
However, in most cases there is no simple way to impose a pump speed w proportional to the liquid flowrate Q.sub.L.
Operating conditions can differ greatly from one installation to another, in terms of:
head losses in the recovery pipe upstream and downstream of the pump, PA1 the possible presence of calibrated valves at the recovery tank which can generate within the latter a pressure different from atmospheric pressure and corresponding to an additional hydraulic resistance on the recovery pipe, PA1 internal leakage of the recovery pump, dependent on the upstream-downstream pressure difference, which affects its efficiency. PA1 head losses in the recovery pipe can vary with time because of: PA1 the internal leakage of the pump can vary because of wear, as in vane pumps, for example. PA1 the density of the vapor varies with the nature of the hydrocarbons and the temperature of the vehicle fuel tanks, which modifies the effect of the upstream and downstream head losses. PA1 the vapor pressure in the recovery tank can also vary with the nature of the hydrocarbons and the temperature. PA1 liquid dispensing means adapted to cause said liquid to flow with a liquid flowrate Q.sub.L between a storage tank and said tank, PA1 vapor recovery means adapted to cause said vapor to flow with a vapor flowrate Q.sub.V along a pipe between said tank and a recovery tank, said vapor flowrate Q.sub.V being controlled by a parameter G characteristic of said recovery means, PA1 establishing an equation EQU G=F(Q.sub.v, {P.sub.i }) PA1 determining an initial value {P.sub.i }.sub.o of the parameters P.sub.i, PA1 on each dispensing k of liquid: PA1 an initial table [p.sub.o.sup.j, Q.sub.V.sup.j ] (j=1, . . . , N) is established linking N values of the parameter p to N values of the vapor flowrate Q.sub.V, PA1 on each dispensing k of liquid:
To summarize, to obtain a given vapor flowrate Q.sub.V, it is necessary to impose on the recovery pump a rotation speed w that depends on the installation.
To allow for the parameters mentioned above it is standard practice to calibrate the complete installation when installed on the site. During this calibration a recovery pump speed w is fixed and the corresponding vapor flowrate Q.sub.V is measured using a flowmeter or a gas meter. A table (w, Q.sub.V) is drawn up in this way relating the speed w and the vapor flowrate Q.sub.V with a sufficient number of points to define the characteristic of the pump under these operating conditions. This table is stored in memory in a microprocessor.
In normal operation, the flowmeter is removed and, during dispensing of hydrocarbons at a liquid flowrate Q.sub.L, the microprocessor looks up in the table the speed w to be imposed on the recovery pump such that Q.sub.V =Q.sub.L.
This prior art recovery method has the following disadvantages, however:
progressive partial blocking with dust, PA2 a change in the cross-section of the elastomer hoses due to the prolonged presence of hydrocarbons. This applies in particular to the part of the pipe upstream of the pump, which generally comprises an elastomer tube surrounded with pressurized liquid, this part representing the core of a coaxial hose. PA2 measuring the liquid flowrate Q.sub.Lk and determining a value G.sub.k of the parameter G to be imposed on the recovery means by the equation: EQU G.sub.k =F(Q.sub.Lk, {P.sub.i }.sub.k-1) PA2 determining a new value {P.sub.i }.sub.k of the parameters P.sub.i to be used for the next dispensing k+1 of liquid. PA2 a value p.sup.j.sub.k-1 of the parameter p is used in the equation EQU G.sub.k =F(Q.sub.Lk, {p.sub.i }.sub.k-1) PA2 the vapor flowrate Q.sub.Vk is measured and a corresponding value p.sub.k of the parameter p is determined, PA2 a coefficient A.sub.k is calculated such that EQU A.sub.k =p.sub.k /p.sup.j'.sub.o with [p.sup.j'.sub.o, Q.sup.j'.sub.V =Q.sub.Vk ] PA2 a new table